#228 - implement basic plumbing for plugins

This functionality is to be considered in 'alpha' state. The aim is to
enable basic extensibility outside of the Packages/Vintageous directory.

To that end, a PluginManager class has been created to gather date needed
to encode commands as Vintageous expects them.

As a sample implementation of a plugin, check out:
https://github.com/guillermooo/Vintageous_Plugin_Surround
#260 - improve pasting linewise onto visual line selection
improve handling of user input; edit changelog
Added tag 1.4.1 for changeset af01059bd5bd